This invention relates to cardiovascular monitoring and, more particularly, to an absorption spectroscopic catheter for the in vivo measurement of blood gas partial pressures as well as blood pressure and pulse rate.
Pressure transducer catheters are well-known (References 1, 2), as are electrolytic type catheters for determining blood gases (3). Various optical catheters have been conceived for the measurement of gas content in blood (4-9) and for providing both blood gases and pressure-pulse rate data (8). Some of such systems utilize fiber optic technology to introduce light in the red-visible region of the spectrum into the bloodstream which is reflected by blood molecules. The reflected light is then colorimetrically analyzed to determline blood color from which information pertaining to oxygen saturation can be derived. This information, however, is actually a ratio of the number of oxygenated hemoglobin molecules to non-oxygenated hemoglobin molecules, and does not provide data in terms of the partial pressure of oxygen which is a vital parameter vis-a-vis the life of the catheterized patient. Another disadvantage of colorimetric systems is that carbon dioxide content in the blood is not directly obtainable.
Other catheter systems utilizing the arts of gas chromotography (10) or mass spectrometry (11) have been devised to measure cardio-vascular functions. Such systems, however, generally require the removal of a blood sample from the body before analysis can take place. The analytical components are very large and are usually located in laboratories which are often far removed from the operating room. Once the blood reaches the laboratory the analysis response time of such systems is typically slow. Delay is a primary disadvantage of mass spectrometric and gas chromotographic systems. Expense is a further disadvantage.
The principles of absorption spectrometry are well-known and find application in a number of analytical systems, procedures and devices. These principles, however, have not been applied to the rapid and accurate in vivo analysis of dissolved gases in blood. Briefly, and in a very much simplified manner, the principle of operation of the present absorption spectrometry catheter system is described as follows.
Each atom or molecule absorbs and radiates electromagnetic radiation in discrete quantitative increments at a number of discrete levels of energy. In the present instance, the electro-magnetic energy is in the energy range referred to as "light," including both the visible and the invisible infrared and ultra-violet regions of the light spectrum. When a light beam of a specific energy level, i.e. wavelength, preferably of only one wavelength, i.e. monochromatic light, is passed through a chamber containing a specific substance which absorbs at that wavelength, the amount of absorbed light, and hence the reduction in the intensity of the light beam, is proportional to the number of atoms or molecules of the substance in the chamber which interact with the incident radiation. The ratio of intensity incident light, I.sub.o, to the intensity of the exit light, I.sub.f, is a measure of the absorbed light and, therefore, a measure of the amount of the substance in the absorption chamber.
Actually, any given substance will absorb light of many differnt energy levels (wavelengths), some wavelengths being strongly absorbed and others much less strongly absorbed. This variation in amount of absorption with wavelength is referred to as the absorption spectrum of the particular material.
When the substance to be measured is a gas, such as oxygen or carbon dioxide, it is convenient to measure the amount of the gas present in a chamber of defined dimensions. According to the gas law, the pressure of the gas in such a chamber is directly proportional to the amount, or number of molecules, of gas in the chamber. Thus, it is possible to measure directly the pressure of a given gas in the chamber simply by measuring the total amount of the gas in the chamber. Where more than one gas is present the pressure contribution of each constituent gas is referred to as the "partial pressure" of that gas.
In any system which includes gases, whether it be a gaseous system, such as a chamber of defined proportions, or a liquid system, such as flowing blood, each gaseous component exerts a pressure proportional to the total amount of the gas in the system. Thus, each gas dissolved in the blood exerts a "partial pressure" in the blood stream. If such a system having a partial pressure of a given gas, for example blood with an oxygen or carbon dioxide partial pressure, is placed in contact with a barrier which is permeable to the gas but not to the blood, the gas will permeate and diffuse through the barrier, i.e. dissolve in one side and out the other, until the partial pressure of that gas on the other side of the barrier equals the partial pressure of the gas in the blood stream. Actually, the pressures on each side of the barrier need not be exactly equal since there are permeation factors, and other factors, which effect the flow through the barrier; however, the gas will flow through the barrier until an equilibrium value is reached at which time the rate of diffusion through the barrier is equal in both directions.
This principle is applied in the present invention by placing a catheter which includes a chamber of defined dimensions in the blood stream. All or part of the wall of the chamber is made of a barrier membrane which is permeable to oxygen and carbon dioxide and/or selected other gases, referred to as a semipermeable membrane. The partial pressure of a given gas in the chamber, at equilibrium, is directly proportional to the partial pressure of gas in the blood. Accordingly, by measuring the partial pressure of the gas in the chamber, by measuring the total amount present as discussed before, the partial pressure of dissolved gas in the blood can be determined. The unique application of these principles in the apparatus and systems and methods of this invention are an important feature of this invention.
Absorption spectroscopy is particularly useful where emission spectra are difficult to obtain due to the high energy levels required to achieve electronic configurations excitations. This is especially true of polyatomic and diotonic gases. For example, absorption spectroscopy has been successfully employed to measure the ozone level of the atmosphere. Since low energy radiation is sufficient for obtaining absorption spectra, measuring systems based on this concept are very advantageous and well suited for use in the in vivo measurement of cardiovascular functions. The development of high infrared transmissive optical fibers has made possible the efficient utilization of the absorption concept in blood catheters. The use of absorption chambers in conjunction with such catheters provides for flexible and accurate monitoring of one or a combination of several blood gases.
The preferred embodiment of the present invention allows for the simultaneous measurement of oxygen and carbon dioxide partial pressures, vital indicators with respect to cardiovascular performance. Furthermore, the same monitor is easily adapted to also measure the equally vital overall blood pressure and pulse rate, thereby embodying a complete, yet convenient and accurate, monitoring device. Convenience in use and mobility of the monitor, because of its small dimensions and reduced space requirements of the optical and electronic components, are important features of the present invention. Use of monochromatic light of strongly absorbed wavelength provides both accuracy and sensitivity for both gases, with minimum effect from the presence of other gases. Response time of the absorption catheter is very short, only about three seconds.